In the OPTIFORD project food intake data were collected in Denmark, Finland, Ireland, Poland and Spain, by means of Food Frequency Questionnaires (FFQ). FFQ is a pre-coded food intake questionnaire, and for each food the question contains several kinds of information. For example for milk it is type of milk ingested, the portion size, and how often it has been drunk. The frequency is generally divided into 9 categories, from 4-5 times per day to less than once per month.
The Food Frequency Questionnaires (FFQ) was not identical in the five countries. Five different languages were used, and as the food consumption pattern is different in different countries, the FFQ did not include exactly the same foods in all countries. In each country each food (each recipe) should have a unique identification. Sometimes the food is a single food, e.g. milk, which can be linked directly to a food composition database. In other instances the foods are complex, e.g. mixed dishes that has undergone some sort of preparation.
The food intake data, i.e. FFQ and data on portion size and recipe information were sent by e-mail to Denmark. The calculations for the estimations of the vitamin D and calcium intake took place at DFVF, Copenhagen. By this method the food intake data from the different countries were handled in the same way, and the intake estimation could be calculated using data either from national food composition databases or from a common database developed during the Optiford project. This made it possible to avoid differences due to different national databases.